System and method for controlling power in a communication system

ABSTRACT

A method and system for controlling power in a communication system are provided, in which an relay station calculates the capacity of a first channel between a base station and the relay station, an mobile station calculates the capacity of a second channel between the relay station and the mobile station, the mobile station notifies the relay station of the capacity of the second channel, and the relay station calculates a power control value according to the capacity of the first channel and the capacity of the second channel.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

The present application claims the benefit under 35 U.S.C. §119(a) of a Korean Patent Application filed in the Korean Intellectual Property Office on Nov. 30, 2006 and assigned Serial No. 2006-119735, the entire disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to a communication system. More particularly, the present invention relates to a system and method for controlling power in a multi-hop communication system.

BACKGROUND OF THE INVENTION

Active studies have been conducted on providing high-speed services with different Quality of Service (QoS) requirements to users in future-generation communication systems. A major future future-generation communication system is Institute of Electrical and Electronics Engineers (IEEE) 802.16e.

Depending on whether a transmitter (e.g., a base station) receives feedback information from a receiver (e.g., a mobile station), power control schemes are classified into open-loop power control and closed-loop power control in communication systems.

Open-loop power control is a power control scheme in which the transmitter decides on the channel status of the receiver independently without receiving feedback information (for example, channel quality information) and controls power accordingly. The open-loop power control scheme is based on the reciprocity of downlink and uplink channels. The channel reciprocity means that the downlink and uplink channels experience similar path attenuation between the transmitter and the receiver. That is, the open-loop power control scheme is a transmission scheme in which the transmitter itself estimates the signal reception quality of the receiver based on the reciprocity between the downlink and uplink channels, calculates a required transmit power based on the estimation, and transmits a signal with the transmit power.

Closed-loop power control is a power control scheme in which the transmitter controls transmit power based on channel information received from the receiver on a feedback channel without determining the channel quality independently, as compared to the open-loop power control. Despite the shortcoming of overhead from the feedback channel, the closed-loop power control scheme can control the power of a transmission signal more accurately than the open-loop power control scheme since the transmitter gets knowledge of the channel quality of the receiver.

Meanwhile, the future-generation communication system adopts a multi-hop relay scheme in order to enable high-speed communications and accommodate a larger number of calls. Since communications are conducted between a fixed base station (BS) and a mobile station (MS) via a direct link, a highly reliable radio communication link can be easily established between them. However, due to the fixedness of the base station, the configuration of a wireless network is less flexible, making it difficult to provide an efficient communication service in a radio environment characterized by a fluctuating change in traffic distribution and required calls. To avert this problem, the communication system employs a multi-hop relay scheme using fixed or mobile relay stations. The multi-hop communication system can reconfigure a network fast according to an environmental change and operate the entire wireless network efficiently. Therefore, a self-configurable radio network can be implemented in the communication system, taking the form of a multi-hop communication system.

The multi-hop communication system expands cell coverage and increases system capacity by multiple hops. When the channel status between a base station and a mobile station is poor, a multi-hop relay path established with the mobile station via relay stations and thus a better radio channel can be provided to the mobile station. Hence, the use of multiple hops enables provisioning of a more efficient communication service to mobile stations in a shadowing area experiencing severe shielding due to buildings. Also, the multi-hop relay scheme provides a high-speed data channel to a mobile station at a cell boundary in a poor channel status and expands cell coverage.

For the base station to transmit data to the mobile station via a relay station (RS) in a multi-hop relay path, a data transmission power control scheme is required for the base station and the relay station. That is, there exists a need for a transmission power control scheme that efficiently utilizes limited channels, for example, a channel between the base station and the RS and a channel between the relay station and the mobile station, and increases data rates in the multi-hop communication system.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is a primary aspect of exemplary embodiments of the present invention to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of exemplary embodiments of the present invention is to provide a system and method for controlling power in a communication system.

Another aspect of exemplary embodiments of the present invention is to provide a system and method for controlling power in a multi-hop communication system.

A further aspect of exemplary embodiments of the present invention is to provide a system and method for controlling power so as to increase channel use efficiency and data rate in a multi-hop communication system.

In accordance with an aspect of exemplary embodiments of the present invention, there is provided a method for controlling power in a communication system having a base station, a mobile station for receiving a communication service from the base station, and a relay station for providing a multi-hop path between the base station and the mobile station, in which the relay station calculates the capacity of a first channel between the base station and the relay station, the mobile station calculates the capacity of a second channel between the relay station and the mobile station and notifies the relay station of the capacity of the second channel, and the relay station calculates a power control value according to the capacity of the first channel and the capacity of the second channel.

In accordance with another aspect of exemplary embodiments of the present invention, there is provided a system for controlling power in a communication system having a base station, a mobile station for receiving a communication service from the base station, and a relay station for providing a multi-hop path between the base station and the mobile station, in which the mobile station calculates the capacity of a second channel between the relay station and the mobile station and notifies the relay station of the capacity of the second channel, and the relay station calculates the capacity of a first channel between the base station and the relay station and calculates a power control value according to the capacity of the first channel and the capacity of the second channel.

Before undertaking the DETAILED. DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:

FIG. 1 illustrates the configuration of a multi-hop communication system according to an exemplary embodiment of the present invention;

FIG. 2 is a flowchart of an operation of a relay station in the multi-hop communication system according to an exemplary embodiment of the present invention; and

FIG. 3 is a flowchart of an operation of a mobile station in the multi-hop communication system according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 3, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication system.

Exemplary embodiments of the present invention provide a method and system for controlling power in a communication system, for example, in a multi-hop communication system. While the exemplary embodiments of the present invention are provided in the context of a multi-hop communication system, it is to be clearly understood that the power control system and method of the present invention are also applicable to other communication systems.

The exemplary embodiments of the present invention also provide a system and method for controlling the transmit power of a base station (BS) and a relay station (RS) when the base station transmits data to a mobile station (MS) in a multi-hop relay path (i.e., via the RS in a multi-hop communication system). The exemplary embodiments of the present invention provide a system and method for controlling transmit power so as to efficiently utilize limited channels, for example, a channel between a base station and a relay station and a channel between the relay station and a mobile station (MS), and increase data rate in a multi-hop communication system. In the exemplary embodiments of the present invention, the transmit power of the base station (BS) and the relay station (RS) is controlled based on the capacities of the BS-RS channel (first channel) and the RS-MS channel (second channel).

FIG. 1 illustrates the configuration of a multi-hop communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 1, the multi-hop communication system includes a base station (BS) 101 for covering a predetermined cell, a mobile station (MS) 105 for receiving a communication service from the BS 101, and a relay station (RS) 103 for relaying between the BS 101 and the MS 105, that is, providing a multi-hop relay path between the BS 101 and the MS 105. A channel between the BS 101 and the RS 103 (i.e., a first channel) is denoted by H₁ 107 and a channel between the RS 103 and the MS 105 (i.e., a second channel) is denoted by H₂ 109.

The RS 103 receives a pilot signal from the BS 101 and calculates the channel capacity of the first channel H₁ 107 by estimating the first channel H₁ 107 as follows:

$\begin{matrix} {C_{1} = {\sum\limits_{n = 1}^{N_{ir}}{{\log_{2}\left( {1 + {{g_{ir}(n)}{P_{ir}(n)}}} \right)}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 1} \right\rbrack \end{matrix}$

In Equation 1, C₁ denotes the capacity of the first channel H₁ 107, N_(ir) denotes the total number of the channels between the BS 101 and the RS 103, i denotes the index of the BS 101, r denotes the index of the RS 103, g_(ir)(n) denotes the gain of channel n among the channels between the BS 101 and the RS 103, that is, the gain of the channel calculated by Singular Value Decomposition (SVD), and P_(ir)(n) denotes the transmit power with which the BS 101 transmits data to the RS 103 on channel n.

The MS 105 receives a pilot signal from the RS 103 and estimates the channel of each antenna by calculating the strength of the pilot signal. The MS 105 calculates a signal-to-interference and noise ratio (SINR) or a carrier-to-interference and noise ratio (CINR) based on the pilot signal strength and generates channel quality information (CQI) corresponding to the SINR or CINR. Then, the MS 105 determines whether to feed back the CQI to the RS 103 on a feedback channel. The case where the MS 105 feeds back the CQI to the RS 103, that is, the RS 103 gets knowledge of the channel status of the second channel H₂ 109 and the case where the MS 105 does not feed back the CQI to the RS 103, that is, the RS 103 has no knowledge of the channel status of the second channel H₂ 109 will be described in great detail.

In the case where the MS 105 feeds back the CQI to the RS 103, it receives a pilot signal from the RS 103 and calculates the capacity of the second channel H₂ 109 by estimating the second channel H₂ 109, as follows:

$\begin{matrix} {C_{2} = {\sum\limits_{n = 1}^{N_{rj}}{{\log_{2}\left( {1 + {{g_{rj}(n)}{P_{rj}(n)}}} \right)}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 2} \right\rbrack \end{matrix}$

In Equation 2, C₂ denotes the capacity of the second channel H₂ 109, N_(rj) denotes the total number of the channels between the RS 103 and the MS 105, r denotes the index of the RS 103, j denotes the index of the MS 105, g_(rj)(n) denotes the gain of channel n among the channels between the RS 103 and the MS 105, that is, the gain of an independent channel calculated by SVD, and P_(rj)(n) denotes the transmit power with which the RS 103 transmits data to the MS 105 on channel n.

After calculating the capacity of the second channel H₂ 109 by Equation 2, the MS 105 transmits feedback information including CQI, C₂ and w₂ on the feedback channel to the RS 103. w₂ denotes a waterfilling level for the second channel H₂ 109.

Upon receipt of the feedback information from the MS 105 on the feedback channel, the RS 103 compares the channel capacity C₁ calculated by Equation 1 with the received channel capacity C₂ and calculates a power control value according to the comparison result. More specifically, if C₁ is larger than C₂, the RS 103 calculates a power control value appropriate for this case and transmits feedback information with the power control value to the BS 101. The BS 101 controls its transmit power for data transmission to the RS 103 based on the power control value.

If C₁ is less than C₂, the RS 103 calculates a power control value appropriate for this case and controls its transmit power for data transmission to the MS 105 based on the power control value. The power control value ΔP is computed by:

ΔP=(2 log 2)·w·ΔC.  [Eqn. 3]

In Equation 3, w denotes a waterfilling level which is a waterfilling level w₁ for the first channel H₁ 107 or the waterfilling level w₂ for the second channel H₂ 109, ΔC denotes the difference between C₁ and C₂. When C₁ is larger than C₂, an operation for calculating ΔP in the RS 103 will be described with reference to Equations 4 and 5.

When C₁ is larger than C₂, ΔC is given as:

ΔC=C ₁ −C ₂,  [Eqn. 4]

and ΔP is computed by:

ΔP=(2 log 2)·w ₁·(C ₁ −C ₂).  [Eqn. 5]

In Equation 5, ΔP denotes a power control value for data transmission from the BS 101 to the RS 103 and w₁ denotes a waterfilling level for the first channel H₁ 107. The RS 103 then transmits feedback information including ΔP to the BS 101. Thus the BS 101 controls the power of data to be transmitted to the RS 103 according to ΔP.

When C₁ is less than C₂, ΔC is given as:

ΔC=C ₂ −C ₁,  [Eqn. 6]

and ΔP is computed by:

ΔP=(2 log 2)·w ₂·(C ₂ −C ₁).  [Eqn. 7]

In Equation 7, ΔP denotes a power control value for data transmission from the RS 103 to the MS 105 and w₂ denotes a waterfilling level for the second channel H₂ 109. The RS 103 then controls the power of data to be transmitted to the MS 105 according to ΔP.

Now a description will be made of the case where the MS 105 does not feed back a CQI to the RS 103. The MS 105 receives a pilot signal from the RS 103 and calculates the capacity of the second channel H₂ 109 by estimating the second channel H₂ 109, as follows:

$\begin{matrix} {C_{2} = {\sum\limits_{n = 1}^{N_{rj}}{{\log_{2}\left( {1 + {{g_{rj}(n)}\frac{\sum\limits_{n = 1}^{N_{rj}}{P_{rj}(n)}}{N}}} \right)}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 8} \right\rbrack \end{matrix}$

In Equation 8, C₂ denotes the capacity of the second channel H₂ 109, N_(rj) denotes the total number of the channels between the RS 103 and the MS 105, r denotes the index of the RS 103, j denotes the index of the MS 105, g_(rj)(n) denotes the gain of channel n among the channels between the RS 103 and the MS 105, and

$\sum\limits_{n = 1}^{N_{rj}}{P_{rj}(n)}$

denotes the average of power allocated to all the channels between the RS 103 and the MS 105.

After calculating the capacity of the second channel H₂ 109 by Equation 8, the MS 105 transmits feedback information including C₂ and a metric for power control of the second channel H₂ 109 between the RS 103 and the MS 105. The metric Q is given as:

$\begin{matrix} {Q = {\sum\limits_{n = 1}^{N_{rj}}{\frac{1}{\frac{N_{rj}}{g_{rj}} + {P_{rj}(n)}}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 9} \right\rbrack \end{matrix}$

In Equation 9, Q denotes a value corresponding to the waterfilling level w₂ for the case where the MS 105 feeds back a CQI to the RS 103 (i.e., for the second channel H₂ 109), N_(rj) denotes the total number of the channels between the RS 103 and the MS 105, r denotes the index of the RS 103, j denotes the index of the MS 105, g_(rj)(n) denotes the gain of channel n among the channels between the RS 103 and the MS 105, calculated by SVD, and P_(rj)(n) denotes transmit power for data transmission from the RS 103 to the MS 105 on channel n.

Upon receipt of C₂ and Q in the feedback information from the MS 105 on the feedback channel, the RS 103 compares the channel capacity C₁ calculated by equation (1) with the received channel capacity C₂ and calculates ΔP. More specifically, if C₁ is larger than C₂, the RS 103 calculates a power control value appropriate for this case and transmits feedback information with the power control value to the BS 101. The BS 101 controls its transmit power for data transmission to the RS 103 based on the power control value.

If C₁ is less than C₂, the RS 103 calculates a power control value appropriate for this case and controls its transmit power for data transmission to the MS 105 based on the power control value. The power control value ΔP is computed by:

$\begin{matrix} {{\Delta \; P} = {\frac{\Delta \; C}{Q}.}} & \left\lbrack {{Eqn}.\mspace{14mu} 10} \right\rbrack \end{matrix}$

In Equation 10, ΔC denotes the difference between C₁ and C₂ and Q denotes the metric for power control of the second channel H₂ 109 between the RS 103 and the MS 105. An operation for calculating ΔP in the case where C₁ is larger than C₂ will be described with reference to, Equations 11 and 12.

When C₁ is larger than C₂, ΔC is given as:

ΔC=C ₁ −C ₂,  [Eqn. 11]

and ΔP is computed by:

ΔP=(C ₁ −C ₂ /Q).  [Eqn. 12]

In Equation 12, ΔP denotes the power control value for data transmission from the RS 103 to the MS 105 and Q denotes the metric for power control of the second channel H₂ 109 between the RS 103 and the MS 105. The RS 103 then transmits feedback information including ΔP to the BS 101. Thus the BS 101 controls the power of data to be transmitted to the RS 103 according to ΔP.

When C₁ is less than C₂, ΔC is given as:

ΔC=C ₂ −C ₁,  [Eqn. 13]

and ΔP is computed by:

ΔP=(C ₂ −C ₁ /Q).  [Eqn. 14]

In Equation 14, ΔP denotes the power control value for data transmission from the RS 103 to the MS 105 and Q denotes the metric for power control of the second channel H₂ 109 between the RS 103 and the MS 105. The RS 103 then controls the power of data to be transmitted to the MS 105 according to ΔP.

FIG. 2 is a flowchart of an operation of the RS in the multi-hop communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 2, the RS receives a pilot signal from the BS and estimates the first channel H₁ in step 201. In step 203, the RS calculates the capacity C₁ of the first channel H₁ by Equation 1. The RS monitors reception of a CQI of the second channel H₂ in step 205. That is, the RS determines whether feedback information received from the MS on the feedback channel includes a CQI.

Upon receipt of the CQI from the MS, (i.e., if the RS has knowledge of the channel status of the second channel H₂), the RS checks C₂ and w₂ included in the feedback information in step 207 and proceeds to step 209.

If the RS has not received the CQI from the MS, that is, the RS does not have knowledge of the channel status of the second channel H₂, the RS checks C₂ and Q included in the feedback information in step 205 and goes to step 209. In step 209, the RS calculates ΔP using C₂ and w₂ or C₂ and Q.

Specifically, if the RS receives the CQI and checks C₂ and w₂, it calculates ΔP by:

$\begin{matrix} {{\Delta \; P} = \left\{ {\begin{matrix} {\left( {2\mspace{11mu} \log \mspace{11mu} 2} \right) \cdot w_{1} \cdot \left( {C_{1} - C_{2}} \right)} & {{{if}\mspace{14mu} C_{1}} > C_{2}} \\ {\left( {2\mspace{11mu} \log \mspace{11mu} 2} \right) \cdot w_{2} \cdot \left( {C_{2} - C_{1}} \right)} & {{{if}\mspace{14mu} C_{1}} < C_{2}} \end{matrix}.} \right.} & \left\lbrack {{Eqn}.\mspace{14mu} 15} \right\rbrack \end{matrix}$

If the RS does not receive the CQI and checks C₂ and Q, it calculates ΔP by:

$\begin{matrix} {{\Delta \; P} = \left\{ {\begin{matrix} {\left( {C_{1} - C_{2}} \right)/Q} & {{{if}\mspace{14mu} C_{1}} > C_{2}} \\ {\left( {C_{2} - C_{1}} \right)/Q} & {{{if}\mspace{14mu} C_{1}} < C_{2}} \end{matrix}.} \right.} & \left\lbrack {{Eqn}.\mspace{14mu} 16} \right\rbrack \end{matrix}$

In step 211, the RS compares C₁ with C₂. If C₁ is larger than C₂, the RS transmits feedback information including ΔP to the BS so that the BS can control the transmit power of data to be transmitted to the RS based on ΔP in step 213.

If C₁ is less than C₂, the RS controls the transmit power of data to be transmitted to the MS based on ΔP in step 217.

FIG. 3 is a flowchart of an operation of the mobile station in the multi-hop communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 3, the MS receives a pilot signal from the RS and estimates the second channel H₂ in step 301 and determines whether to transmit a CQI to the RS on the feedback channel in step 303. If the MS determines to transmit the CQI to the RS, it calculates the CQI of H₂ and C₂ by Equation 2 in step 305 and transmits feedback information including the CQI, C₂ and w₂ to the RS on the feedback channel in step 307.

If the MS determines not to transmit the CQI to the RS, it calculates C₂ by Equation 8 and Q by Equation 9 in step 309 and transmits feedback information including C₂ and Q to the RS on the feedback channel in step 311.

As is apparent from the above description, the present invention advantageously increases channel use efficiency and data rate by controlling power in accordance with channel capacities between a BS and an RS and between the RS and an MS in a multi-hop communication system.

While the invention has been shown and described with reference to certain exemplary embodiments of the present invention thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents.

Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims. 

1. A method for controlling power in a communication system having a base station, a mobile station for receiving a communication service from the base station, and a relay station for providing a multi-hop path between the base station and the mobile station, the method comprising: calculating the capacity of a first channel between the base station and the relay station by the relay station; calculating the capacity of a second channel between the relay station and the mobile station by the mobile station; notifying the relay station of the capacity of the second channel by the mobile station; and calculating a power control value according to the capacity of the first channel and the capacity of the second channel by the relay station.
 2. The method of claim 1, wherein the power control value calculation comprises: comparing the capacity of the first channel with the capacity of the second channel by the relay station; calculating the power control value and transmitting the power control value to the base station by the relay station, if the capacity of the first channel is larger than the capacity of the second channel; and controlling transmit power for transmission to the relay station based on the power control value by the base station.
 3. The method of claim 1, wherein the power control value calculation comprises: comparing the capacity of the first channel with the capacity of the second channel by the relay station; and calculating the power control value and controlling transmit power for transmission to the mobile station based on the power control value by the relay station, if the capacity of the first channel is less than the capacity of the second channel.
 4. The method of claim 1, wherein the calculation of the capacity of the second channel comprises calculating the capacity of the second channel by estimating the second channel from a signal received from the relay station by the mobile station.
 5. The method of claim 4, wherein the calculation of the capacity of the second channel further comprises: measuring channel status of the estimated second channel by the mobile station; and generating channel quality information corresponding to the channel status by the mobile station.
 6. The method of claim 5, wherein the notification comprises transmitting feedback information including the capacity of the second channel, the channel quality information, and a waterfilling level for the second channel to the relay station by the mobile station.
 7. The method of claim 6, wherein the power control value calculation comprises: comparing the capacity of the first channel with the capacity of the second channel included in the feedback information by the relay station; calculating the power control value based on the capacity of the second channel and waterfilling level for the second channel by the relay station, if the capacity of the first channel is less than the capacity of the second channel; and controlling transmit power for transmission to the mobile station based on the power control value by the relay station.
 8. The method of claim 4, wherein the calculation of the capacity of the second channel further comprises calculating a metric for the estimated second channel by the mobile station.
 9. The method of claim 8, wherein the notification comprises transmitting feedback information including the capacity of the second channel and the metric for the second channel to the relay station by the mobile station.
 10. The method of claim 9, wherein the power control value calculation comprises: comparing the capacity of the first channel with the capacity of the second channel included in the feedback information by the relay station; calculating the power control value based on the capacity of the first channel and a waterfilling level for the first channel and transmitting the power control value to the base station by the relay station, if the capacity of the first channel is larger than the capacity of the second channel; and controlling transmit power for transmission to the relay station based on the power control value by the base station.
 11. The method of claim 9, wherein the power control value calculation comprises: comparing the capacity of the first channel with the capacity of the second channel included in the feedback information by the relay station; calculating the power control value based on the capacity of the second channel and the metric for the second channel by the relay station, if the capacity of the first channel is less than the capacity of the second channel; and controlling transmit power for transmission to the mobile station based on the power control value by the relay station.
 12. A system for controlling power in a communication system having a base station, a mobile station for receiving a communication service from the base station, and a relay station for providing a multi-hop path between the base station and the mobile station, the system comprising: the mobile station for calculating the capacity of a second channel between the relay station and the mobile station and notifying the relay station of the capacity of the second channel; and the relay station for calculating the capacity of a first channel between the base station and the relay station and calculating a power control value according to the capacity of the first channel and the capacity of the second channel.
 13. The apparatus of claim 12, wherein the relay station compares the capacity of the first channel with the capacity of the second channel, and if the capacity of the first channel is larger than the capacity of the second channel, calculates the power control value and transmits the power control value to the base station.
 14. The apparatus of claim 12, wherein the relay station compares the capacity of the first channel with the capacity of the second channel, and if the capacity of the first channel is less than the capacity of the second channel, calculates the power control value and controls transmit power for transmission to the mobile station based on the power control value.
 15. The apparatus of claim 12, wherein the mobile station calculates the capacity of the second channel by estimating the second channel from a signal received from the relay station.
 16. The apparatus of claim 15, wherein the mobile station measures channel status of the estimated second channel and generates channel quality information corresponding to the channel status.
 17. The apparatus of claim 16, wherein the mobile station transmits feedback information including the capacity of the second channel, the channel quality information, and a waterfilling level for the second channel to the relay station.
 18. The apparatus of claim 17, wherein the relay station compares the capacity of the first channel with the capacity of the second channel included in the feedback information, calculates the power control value based on the capacity of the second channel and waterfilling level for the second channel by the relay station, if the capacity of the first channel is less than the capacity of the second channel, and controls transmit power for transmission to the mobile station based on the power control value.
 19. The apparatus of claim 15, wherein the mobile station calculates a metric for the estimated second channel.
 20. The apparatus of claim 19, wherein the mobile station transmits feedback information including the capacity of the second channel and the metric for the second channel to the relay station.
 21. The apparatus of claim 20, wherein the relay station compares the capacity of the first channel with the capacity of the second channel included in the feedback information, and calculates the power control value based on the capacity of the first channel and a waterfilling level for the first channel and transmits the power control value to the base station by the relay station, if the capacity of the first channel is larger than the capacity of the second channel, and the base station controls transmit power for transmission to the relay station based on the power control value.
 22. The apparatus of claim 20, wherein the relay station compares the capacity of the first channel with the capacity of the second channel included in the feedback information, calculates the power control value based on the capacity of the second channel and the metric for the second channel by the relay station, if the capacity of the first channel is less than the capacity of the second channel, and controls transmit power for transmission to the mobile station based on the power control value. 